392 F. K Wright— Bi-quartz Wedge Plate. 



normal to the principal plane of the analyzer and the bisector 

 of the angle between the nicols of the Lippich polarizer can be 

 readily compensated by a slight shift of the zero in the quartz 

 compensator scale. This apparatus is easily superior to the 

 ordinary quartz compensating polarimeter and is of more 

 general application, but it is mechanically difficult and expen- 

 sive to build and its adjustments are rather sensitive to wear 

 and tear. It therefore occurred to the writer that the bi-quartz 

 wedge plate described in the foregoing paper might serve the 

 same purpose much more simply and therefore be of some prac- 

 tical utility in polarimetric and saccharimetric measurements. 

 By means of this wedge the plane of vibration of waves from 

 the analyzer is made to rotate from 0° to any specified angle 

 by varying the thickness of the wedge employed, the rotation 

 of the one half being right-handed and the other left-handed. 



Since the problem of saccharimetry from the practical 

 standpoint reduces to the determination of the exact angle of 

 rotation of the plane of polarized light after its passage through 

 the sugar solution, or, in brief, to the accurate setting of the 

 analyzer or the compensating system with the analyzer, it is 

 apparent that the bi-quartz wedge plate is directly applicable. 



The mode of application is the same whether a monochro- 

 matic or a white light source is used. If monochromatic light is 

 employed a feasible arrangement of the apparatus is shown in 

 diagram in fig. 1. The monochromatic light, properly restricted 

 in wave length and as intense as possible, is polarized by the 

 nicol prism P. On passing through the solution T its plane of 

 polarization is rotated and the angle of its rotation is determined 

 by revolving the analyzer (fitted with an accurately divided 

 degree circle C) until darkness ensues. The exact position of 

 extinction is then found by inserting the bi-quartz wedge, 

 which offers opportunity for a finer adjustment, and ascertain- 

 ing that the intensity of illumination of both halves of the 

 wedge is precisely equal for all positions of the wedge plate. * 



* The wedge plate should be mounted in a metal frame arranged to slide 

 like the quartz compensator in accurate grooves. The quartz wedge plate 

 used in the examination of extinction angles in crystals is 35 ram long, 

 12 mm wide, and is made up of wedges 0'3 mm thick at one end and0*5 mm at 

 the other, which are underlain by two quartz plates each •35 mm thick. The 

 rotary angle range in each half is from — 1 - 1° to +3*2°. 



For general polarimetric work, in view of the limited sensitiveness of the 

 eye and the difficulty in obtaining homogeneous illumination of sufficient 



a 

 intensity, it is desirable that the angle ^ should be capable of being increased 



to 15°. To accomplish this in a single wedge it would be best to make it 

 50mm long, 10 or 12 ram wide with the thin end of each wedge 35 mm and the 

 thick end l'lO 1 "™ in thickness, combined with quartz plates 0"4 mm thick. 

 This gives a pitch to the wedge of 1*5 : 100 or 8°. The point of zero deflection 

 or the position of the black band will fall 3'3 mm from the thin end and the 

 deflection on the thick end will be ± 15° for sodium light. The total thick-' 

 ness of the wedge plate is l\5 mm . Canada balsam should be used for the 

 cementing material and care must be taken to have the wedge surfaces adja- 

 cent to the Canada balsam and also the two surfaces of the complete wedge 

 plate parallel. 



